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Impacts to the Ecosystem

Image depicting the interconnectedness of a sagebrush ecosystemThe components of an ecosystem are really pretty basic. The main players are plants, animals, watersheds, and climate. However, processes such as energy flow and nutrient and water cycling make ecosystems incredibly complex. Embedded within the ecosystem concept is the idea of interconnectedness. Weed introductions often alter ecosystem processes, creating a domino effect throughout the rest of the ecosystem. Let’s try to understand how such a simple thing as a new plant species can have such far-reaching effects.

Energy Flow
Energy flow refers to the transfer of energy between different living organisms and their environment. This is somewhat the same as a food web. Through photosynthesis, plants use energy from the sun to grow and reproduce. When plants are eaten, the consumers use the energy stored in plants for their own movement and growth. Carnivores in turn eat other animals, using the energy stored in their prey. When all living organisms die, microorganisms decompose their tissues, utilizing the available energy. Changes in the plant community may alter energy flow by disrupting one or more steps in the process.

Nutrient Cycling
Picture of mineral soilUnlike energy, many nutrients come from the mineral soil. Others, like nitrogen, can also from the atmosphere. Many nutrients are often limited and are recycled as organisms grow, shed leaves, and die. Cheatgrass invasion can affect the rate at which this cycling occurs. For example, nutrient availability in native shrub steppe ecosystems is characteristically low. This is partly because the perennial plants keep nutrients in their tissues for a long time. With the loss of perennial plants, nutrients return to the soil. Cheatgrass thrives with more available nutrients, especially nitrogen. In addition, cheatgrass litter is decomposed quickly compared to native vegetation. This combination generally means faster nutrient cycling, and may make restoring diverse native plant communities difficult.

Picture of a mountain, foothills, and valleyWater Cycling
A watershed is the area of land drained by a common stream. Vegetation affects how a watershed functions. In the Great Basin, perennial grasses and shrubs protect the soil against erosion. When native plants are replaced by invasive weeds, a watershed can change. Cheatgrass and other annual invaders have shallow roots which don’t hold the soil together as well as deep-rooted perennials. Also, because cheatgrass burns frequently, the soil surface is exposed and erodes easier. A watershed should store moisture from winter months and slowly release it in streams and springs. By altering the soil, weed invasion can decrease infiltration and soil moisture holding capacity and increase runoff, erosion, and flooding.

Image depicting a degenerate sagebrush ecosystem

Biological Diversity
Plants provide food and cover for wildlife. Weed invasions often reduced native plant cover and decrease local plant diversity. This ultimately results in the degradation or loss of native wildlife habitat, and eventual impact to wildlife diversity.

-Food web items:Sagebrush Clips © 2005 Zackery Zdinak
-Soil and mountan: Southwest Regional Gap Analysis Project. Field site database. (http://earth.gis.usu.edu/swgap/trainingsites.html, 26 September 2006).

Text references:
-Booth, M.S., M.M. Caldwell, and J.M. Stark. 2003. Overlapping resource use in three Great Basin species: implications for community invasibility and vegetation. Journal of Ecology Vol. 91 pp. 36-48.
-Entwistle, P.G., A.M. DeBolt, J.H. Kaltenecker, and K. Steenhof, compilers. 2000. Proceedings: Sagebrush Steppe Ecosystems Symposium. Bureau of Land Management Publication No. BLM/ID/PT-001001+1150,Boise, Idaho, USA.
-Norton, J.B. et.al. 2004. Soil morphology and organicmatter dynamics under cheatgrass and sagebrush-steppe plant communities. Journal of Arid Environments Vol. 57 pp. 445-466.
-Pellant, M. 1996. Cheatgrass: the invader that won the west. Unpublished report. On file with: U.S. Department of Agriculture, Forest Service; U.S. Department of Interior, Bureau of Land Management; Interior Columbia Basin Ecosystem Management Project, 112 E. Poplar, Walla Walla, A 99362.